Emiliano V. Aralar

The effects of water hardness on the hatching and viability of silver carp (Hypophthalmichthys molitrix) eggs

Abstract An investigation was conducted to establish water hardness concentration for optimum hatching of silver carp ( Hypophthalmichthys molitrix ) eggs. Eggs were incubated for 19 h at 26.5°C at six levels of water hardness: 100, 200, 300, 400, 500 and 600 mg/l CaCO 3 . Water absorption at 100–200 mg/l CaCO 3 caused eggs to burst prematurely and minimal water absorption occurred at 600 mg/l CaCO 3 . Chloride concentration at 0 and 6 h post-fertilization was significantly related to egg hatchability. Total ammonia-nitrogen at 6 h, pH at 6 h, magnesium hardness at 18 h and chlorides at 18 h significantly influence viability of larvae. A water hardness of 300–500 mg/l CaCO 3 is recommended for the successful hatching of silver carp eggs.

Microcystins in natural blooms and laboratory cultured Microcystis aeruginosa from Laguna de Bay, Philippines.

Laguna de Bay, the largest freshwater lake in the Philippines, experiences periodic blooms of the cyanobacteria Microcystis aeruginosa. Blooms of these cyanobacteria in 1996, 1998 and 1999 were sampled. HPLC and MALDI-TOF mass spectrometry were used to analyze for microcystins. A total of 16 structural variants of the toxin were isolated from the samples with microcystin LR (MC-LR) as the most abundant variant in the samples from 1996 and 1999 making up 77 to 85% of the total, respectively. MC-RR was the dominant variant in the 1998 bloom making up 38%. The samples from 1996 had the highest total toxin concentration (4049 microg g(-1)) followed by those from 1998 (1577 microg g(-1)) and 1999 (649 microg g(-1)). A strain of M. aeruginosa previously isolated from the lake was also cultured in the laboratory under different nitrogen concentrations (1, 3 and 6 mg L(-1)) and elevated phosphorus concentration (0.5 mg L(-1)) to determine the influence of these factors on toxin production. A total of 9 different structural variants of microcystin were isolated from the laboratory cultures with MC-LR consisting more than 75% of the total in all treatments. No significant differences in the total toxin concentration as well as the % distribution of the different variants among treatments were observed. However, the strain of M. aeruginosa cultured in the laboratory had from 3 to 20 times higher total microcystin than those harvested from the lake.

Effects of low nitrogen-phosphorus ratios in the phytoplankton community in Laguna de Bay, a shallow eutrophic lake in the Philippines

The effects of low nitrogen-phosphorus ratios on microalgae from a large eutrophic freshwater lake in the Philippines were investigated. Natural microalgal populations from Laguna de Bay, the largest lake in the Philippines, were cultured using three different nitrogen-phosphorus weight ratios (2N:1P; 6N:1P and 12N:1P) at two phosphorus concentrations (0.25 and 0.5 mg l–1) in each case. The growth and genera composition of the cultures under the different treatments were followed for a 12-week period. Community level responses were assessed based on species richness (s), Shannon-Wiener Index (H’), Simpson Index (λ) and Evenness (J’). Among the different microalgal groups, only the chlorophytes showed a significantly higher density in response to the 12N:1P treatment at the higher P concentration, indicating that the nutrient ratio had a significant interaction with the nutrient levels used in the experiments. The genera found in the different treatments were generally similar; however, the degree of dominance of some varied with treatment during the experiment. The succession of dominant genera also differed among the N:P treatments. The diatoms like Fragilaria, Aulacoseira (= Melosira) and Nitzschia dominated the lowest N:P. On the other hand, chlorophytes (Kirchneriella and Scenedesmus) dominated the highest N:P treatment, particularly from the second to the seventh week of the experiments with the diatoms becoming co-dominant only towards the eighth week until the end of the experimental. The 6N:1P treatment showed a mixed dominance between the diatoms and the chlorophyte genera. The various indices of diversity indicate significantly lower diversity only in the 12N:1P at 0.5 mg l–1 P and not in 12N:1P at 0.25 mg l–1 P.

Resistance to a heavy metal mixture in Oreochromis Niloticus progenies from parents chronically exposed to the same metals

Abstract Adult Oreochromis niloticus were mass spawned in concrete tanks. The one-month old progenies (F 1 ) were exposed for two months to a mixture of 0.01 mg L −1 Hg, 0.1 mg L −1 Cd and 1.0 mg L −1 Zn. The survivors were grown to sexual maturity in a natural environment (lake). The fish were spawned and the progenies (F 2 ) of the exposed F 1 (EF 1 ) were exposed to another mixture of the three metals: 3.0 mg L −1 Zn, 0.30 mg L −1 Cd and 0.01 mg L −1 Hg, both in a static and static-renewal system. Another group of F 2 from unexposed F 1 (UF 1 ) received the same treatment. Results showed that in both exposure systems, survival of the F 2 of EF 1 was significantly higher (P 1 . The medial lethal time (LT 50 ) of the F 2 s were estimated from the time-response curve following regression analysis: 5.16 days (F 2 of UF 1 ) and 9.03 days (F 2 of EF 1 ) in the static exposure experiment; 3.34 days (F 2 of UF 1 ) and 5.52 days (F 2 of EF 1 ) in the static-renewal run. Exposure of the parental stock resulted in the culling out of individuals which were more susceptible to the heavy metals. The more resistant members of the population (survivors) which have the ability to adapt to the toxicants were able to pass on the resistance to their offspring. The results are supported by other studies in the field which demonstrate high resistance in populations of organisms living in contaminated sites.